The disclosures herein relate generally to orthopedic implants and more particularly to femoral hip stems.
To fit into the intrameduallary canal of a human femur, an orthopedic femoral hip stem is characterized by a bulky proximal body and an elongated distal portion. A femoral hip stem is conventionally fabricated as one solid piece, which causes two problems.
First, the proximal body imposes increased manufacturing and material cost due to its complex shape and large size relative to the distal portion of the stem, as well as requiring additional processes to fabricate a surface texture or porous coating on the proximal body for non-cemented fixation. The current trend is to use implants with a porous or textured surface so as to encourage bone ingrowth, allowing for long term fixation of the implant in the bone. This trend arose because the cement that is used to fix the implant in the bone begins to lose its adhesive capacity over time, and therefore, leads to wear debris within the joint. However, additional processes, and consequently additional costs, are required to give the proximal body a porous coating or surface.
Second, the solid metallic proximal body has an elastic modulus much higher than those of surrounding cancellous and cortical bone, leading to bone stress shielding and consequently, resorption. When a femoral stem is implanted, it changes the mode in which stress is applied to the bone. A metallic implant shields the bone from its normal stress by supporting the load that is normally supported by the bone. In addition, rather than the stress from the pelvis being applied directly to the bone, it is applied to the implant which, in turn, transmits the stress to the wall of the bone from inside the intramedullary canal. Because the bone does not carry the mechanical stress in the way that it normally would, the bone can resorb over time, causing a thinning of the cortical wall. Consequently, the implant can become loose and cause pain to the patient.
Some of the known devices for remedying the problems mentioned above include U.S. Pat. No. 4,878,919 which discloses an artificial hip endo-limb comprising a spherical joint supported by a shaft. A porous shell of metal, synthetic material, ceramics and the like is provided on the circumference of the shaft. The porous shell is transversely divided and separated by spacing elements and may be longitudinally divided. The surface can be provided with grooves.
U.S. Pat. No. 5,236,457 and U.S. Pat. No. 5,713,410 describe methods of fabricating one-piece implants with a porous surface using a mold. The former discloses an implant comprised of a plastic body and a metallic, porous surface securely fixed to the body. The implant is made by first producing a porous mold insert having a porous metal structure and a soluble filler material filling a portion of its pores. The mold insert is then placed in a mold and plastic is inserted into the mold and the exposed pores of the insert to form the implant body and securely attach the body and insert. The implant is then contacted by a solvent which dissolves the filler material to expose that portion of the insert which had been filled.
The latter describes an implant having on at least a portion of its exterior surface, an integral, as-cast macrotextured surface having pores with undercut edge profiles. The surface is able to be formed on the implant by a modified casting process. As part of a casting process, positive models of the implants to be cast, or parts thereof, are formed by stereolithographic techniques. Cavities or molds, representing negative images of the implants to be cast, are then formed by encasing one or more models in a refractory material. The positive models are then extracted by heating and thus melting the material from which they are made. Thereafter, molten casting material can be poured into the resulting mold to obtain the implants.
U.S. Pat. No. 5,480,449 describes a method for minimizing a modulus mismatch by making a composite stem for a prosthesis by providing a tapered metal core with a predetermined cross section and a composite shell including a tapered cavity having a cross sectional configuration to receive the metal core. Adhesive is applied to the metal core and the core is placed in the cavity and pressure is applied along the axis of the metal core toward the smaller end of the core, to force the core against the composite shell. Heat is applied with the pressure to bond the composite shell to the metal core. Therefore, what is needed is an orthopedic implant and a cost effective method for fabricating the implant that minimizes the mismatch between the modulus of the implant and the modulus of the surrounding bone material and allows for a porous surface.